The invention is related to particulate precursor compounds for manufacturing a lithium transition metal (M)-oxide powder usable as an active positive electrode material in lithium-ion batteries. More in particular, the precursors may be hydroxide or oxyhydroxide compounds having physical characteristics such as tapped density, specific surface area and median particle size that are related by a mathematical formula to the Ni content of the precursor.
Due to their high energy density, rechargeable lithium and lithium-ion batteries can be used in a variety of portable electronics applications, such as cellular phones, laptop computers, digital cameras and video cameras.
Commercially available lithium-ion batteries typically consist of graphite-based anode and LiCoO2-based cathode materials. However, LiCoO2-based cathode materials are expensive and typically have a relatively low capacity of approximately 150 mAh/g.
Alternatives to LiCoO2-based cathode materials include LNMCO type cathode materials. LNMCO means lithium-nickel-manganese-cobalt-oxides. The composition is LiMO2 or Li1+x′M1−x′O2 where M=NixCoyMnzAm (which is more generally referred to as “NMC”, A being one or more dopants). LNMCO has a similar layered crystal structure as LiCoO2 (space group r-3m). The advantage of LNMCO cathodes is the much lower raw material price of the composition M versus pure Co. The addition of Ni gives an increase in discharge capacity, but is limited by a decreasing thermal stability with increasing Ni content. In order to compensate for this problem, Mn is added as a structural stabilizing element, but at the same time some capacity is lost. Typical cathode materials include compositions having a formula Li1+x(Ni0.51Mn0.29Co0.20)1−xO2 (for example x=0.00-0.03, referred to as NMC532), Li1+x(Ni0.38Mn0.29Co0.33)1−xO2 (for example x=0.08-0.10, referred to as NMC433), Li1+x(Ni0.6Mn0.2Co0.2)1−xO2 (X=0.02-0.04, referred to as NMC622) or Li1+x(Ni0.35Mn0.32Co0.33)1−xO2 (for example x=0.06-0.08, referred to as NMC111). The target lithium-containing composite oxide is generally synthesized by mixing a nickel-cobalt-manganese composite (oxy-)hydroxide as a precursor material (having the same metal composition as the final cathode material will have) with a lithium compound and fired, and the cell characteristics can be improved by substituting a part of nickel, cobalt and manganese by other metal elements. As other metal elements, Al, Mg, Zr, Ti, Sn and Fe are exemplified. The suitable substituting quantity is 0.1 to 10% of the total quantity of the nickel, cobalt and manganese atoms.
Generally, for the production of cathode materials with complex compositions, special precursors such as mixed transition metal hydroxides are used. The reason is that high performance Li-M-O2 needs well mixed transition metal cations. To achieve this without “oversintering” (high temperature sintering for a longer period together with a lithium precursor, typically Li2CO3 or LiOH) the cathode precursors need to contain the transition metal in a well-mixed form (at atomic level) as provided in mixed transition metal hydroxides, carbonates etc. Mixed hydroxides or carbonates are typically prepared by precipitation reactions. Precipitation of mixed hydroxides (for example, the precipitation of a flow of NaOH with a flow of M-SO4 under controlled pH) or mixed carbonates (for example, the precipitation of a flow of Na2CO3 with a flow of M-SO4) allows precursors of suitable morphology to be achieved.
According to U.S. Pat. No. 7,384,706 for example there are provided nickel-cobalt-manganese composite oxyhydroxide particles, formed by precipitating a nickel-cobalt-manganese composite hydroxide. For the precipitation, an aqueous solution of a nickel-cobalt-manganese salt, an aqueous solution of an alkali-metal hydroxide and an ammonium-ion donor are continuously or intermittently supplied to a reaction system, at a temperature between 30 and 70° C., and maintaining the pH at a substantially constant value within a range between 10 and 13; and making an oxidant act on the composite hydroxide. The obtained precursors typically have a median particle size D50 of 3 to 15 μm, a pressed density of more than 2 g/cm3 and a specific surface area (BET value) of 4 to 30 m2/g. From this patent, and also from U.S. Pat. No. 7,585,435, aiming at supplying high density cobalt-manganese co-precipitated nickel hydroxide, it can be understood that the processes for manufacturing transition metal (oxy-)hydroxide precursors are relatively versatile and capable of providing specific values for physical characteristics such as specific surface area (BET), tap density and particle size distribution, by fine-tuning the precipitation processes.
For characterizing a secondary lithium cell one of the most important parameters besides the discharge capacity is the irreversible capacity, which is responsible for the fading of the capacity during cycling. Lithium-excess layered transition metal oxides Li1+xM1−xO2 often have a huge irreversible capacity loss associated with the oxygen and lithium loss from the host structure of the layered oxide at the end of the first charging process. Although the irreversible capacity loss can be significantly reduced by coating with insulating materials (e.g., Al2O3 or MgO), the high surface area associated with the nanostructured lithium layered oxides could have such a high surface reactivity to induce side reactions between the electrodes and the electrolyte. This could lead to destabilization of the active materials and an increase in impeding passivation. Therefore, the electrolyte safety is of major concern, and ways have to be found to eliminate the side reactions and lower the irreversible capacity Qirr. As described by Lu and Dahn in “Layered Li[NixCo1-2xMnx]O2 Cathode Materials for Lithium-Ion Batteries”, Electrochemical and Solid-State Letters, 4 (12) A200-A203 (2001), for x=¼ and ⅜, when cycling between 2.5 and 4.4 V at a current of 40 mA/g, an irreversible capacity loss of 12% is quite acceptable.
In patents such as U.S. Pat. No. 8,268,198 the relationship between the chemical composition of the precursor compound (i.e. the sulfate content) and the irreversible capacity of the lithium transition metal oxide cathode material has been established. A direct relationship between the physical characteristics of the precursor and the irreversible capacity of the lithium transition metal oxide cathode material, wherein also the Ni content of the material is taken into account, has not yet been provided.
The present invention aims to provide improved precursors of lithium transition metal cathode materials for positive electrodes having an intermediate to high Ni content, made by a cheap process, and having a reduced irreversible capacity Qirr upon cycling in the secondary battery.